Our long term objective is to understand the role of endogenous murine retro-virus-related genes in both normal cell processes and neoplastic transformation. Recently, we characterized the endogenous ecotropic proviruses carried by many inbred, recombinant inbred, congenic, and mutant mouse strains by Southern blotting and hybridization with an ecotropic murine leukemia virus (MuLV) DNA specific probe. In this study, we found the dilute (d) and lethal yellow (Ay) mutant coat-color alleles are closely associated with the site of integration of an ecotropic provirus. A spontaneous DBA/2J d allele revertant d+2J) did not contain ecotropic-specific MuLV DNA sequences, suggesting that the dilute mutation may be causally related to virus integration. (1) To further investigate the effect of these two ecotropic proviruses on host genes and their potential role in mutagenesis, we will molecularly clone the virus and flanking cellular DNA sequences associated with these two mutations as well as the empty integration site in the DBA/2J d+2J revertant. We will then use these cellular DNA sequences as hybridization probes to characterize the RNAs transcribed from the cellular DNA sequences associated with the mutant, revertant, and wild type alleles at the d locus and mutant and wild type alleles at the Ay locus. (2) The mechanism(s) by which viral DNA sequences are lost in d allele revertants will be further studied (a) by determining if new d allele revertants (1-2/year/300,000 DBA mice) have concordantly lost MuLV DNA sequences associated with the d locus and (b) by sequencing the empty integration site in d+2J revertant DNA. (3) Expression of most endogenous ecotropic MuLV genomes appears to be under stringent cellular control. We will use DNA infectivity assays of the molecularly cloned DBA/2J ecotropic provirus to further examine the mechanism(s) regulating engodenous ecotropic viral gene expression. (4) Finally, we will continue our studies on the chromosomal location and structure of endogenous ecotropic proviruses.